High performance photoresistor



Sept. 21, 1965 Y. T. SIHVONEN ETAL HIGH PERFORMANCE PHOTORESISTOR FiledDec. 23, 1965 /o RESPONSE RESPONSE I00% 25 fc 2 Sheets-Sheet 1 Fig. I

X MICRONS l Orr PRIOR ART Yro II Sihvonen Sidney G. Parker INVENTORS p1965 Y. T. SIHVONEN ETAL 3,208,022

HIGH PERFORMANCE PHOTORESISTOR 2 Sheets-Sheet 2- Filed Dec. 25, 1963 YroI. Sihvonen Sidney G. Parker INVENTOR S I 0.39 I mm I l I I0 I0 I00 WILLUMINATION f.c.

United States Patent 3,208,022 HIGH PERFORMANCE PHOTORESISTOR Yro T.Sihvonen, Richardson, and Sidney G. Parker,

Dallas, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex.,a corporation of Delaware Filed Dec. 23, 1963, Ser. No. 332,505 15Claims. (Cl. 338-15) The photosensitivity of various semiconductivecompounds such as cadmium sulfide and selenide, as well as the telluridethereof, both in monocrystalline as well as polycrystalline form is wellknown. These photoconductive materials have been utilized in manycommercial devices and their basic photoconductive mechanisms are fairlywell established. The photoconductive elfect may be defined as thechange of electrical conductivity of the material in response tovariations in the intensity of incident radiation. The particular rangeof wavelengths of radiation to which any given photosensitive materialrcsponds is a specific property of the material per se. For cadmiumsulfide and cadmium selenide, this photosensitive range includes asubstantial portion of the visible spectrum. For this reason, theiremployment in the field of photoconductive semiconductor devices andelements is particularly advantageous.

Photocells have been developed whose action is based on the internalphotoelectric eifect occurring in certain semiconductor materials whichin darkness are poor conductors. Electrodes are normally fitted to adisc or block of a substance of this kind and depending on the nature ofthese electrodes, one may obtain a photoconductive cell, or morecommonly referred to as a photoresistor. In other words, a device isproduced whose electrical conductivity varies with the intensity of theincident light. This is contrasted to a photovoltaic effect which is adevice across which dilferences of electrical potential arise when lightfalls upon it. The nature of the photoconductivity in the materials ofthe instant invention, which relates to the II-VI and III-V compounds,can be understood by considering that in a nonconductive solid allelectrons are bound to the ions or atoms which comprise the crystallattice. The free electrons that usually exist in metals and otherconductors are absent or, if present, are in very small numbers. Thislack of free electrons is also applicable to photoconductive materialssuch as cadmium sulfide and cadmium selenide when they are not exposedto radiation of the appropriate wavelength. However, when radiationfalls on the crystal, the energy of the radiation is absorbed within thelattice. Through the action of this radiation, a number of electronsthen become free to move about so that the substance ceases to be aninsulator and then converts to a conductor.

' Photoconductivity is a process which involves the absorption of energyfrom light quanta and provides for the excitation of charge carriersfrom a nonconducting ground state to a higher energy state where thesecharge carriers are free to contribute to the electricalconductivity,'and it also involves the return of these generated chargecarriers from the conducting state to their ground state. In otherwords, photoconductivity processes involve the transition of electronsbetween different energy states within a crystalline material. Adetailed discussion of the various electron processes involved inphotoconductivity is provided by Bube, Photo Conductivity of Solids,John Wiley & Sons, New York, 1960.

Cadmium sulfide and cadmium selenide have been 3,208,022 Patented Sept.21, 1965 E Ice.

found especially useful as photoconductive devices for employment aslight amplifiers and image converters as well as other devices.Considerable activity has been expended into increasing the photoresponse of these materials, and for obtaining new means for providingincreased surface areas. The most common method presently employed isthe use of powdered layers that have been appropriately sintered toefiect bonding. Other methods include various types of powder dielectricmaterial imbedment or the use of evaporated thin films. Sintered layersare preferred for device fabrication inasmuch as they possess superiorphotosensitivity, high mechanical strength, uniformity of response, aswell as ease of formation. While it is well established that thephotoconductive properties are directly dependent on the composition andgeometry of the unit, it is extremely difiicult to provide usefulparameters involving fast response and high photocurrents by followingtechniques disclosed and suggested within the prior art. Photocell risetime and linearity of log resistance versus log light intensity areespecially difficult to obtain in accordance with prior art teachings.Furthermore, the range of resistance from dark to light are incompatiblylow in the photoconductors of the prior art.

It is an object of this invention to overcome the foregoing and relateddisadvantages. More specifically, an object is to provide forsemiconductor devices having a high photosensitivity and a fast responsetime, as well as a near linear relationship between log resistance andlog light intensity, and being highly responsive to light in the visibleand near visible part of the spectrum.

Another object of this invention is to provide for a superior cadmiumsulfide-cadmium selenide photoresistor of high performance.

A still further object of this invention is to make available aneconomical photoresistor having a rise time of less than fivemilliseconds in response to a change in illumination of 25 ft. candles,or less.

To achieve the above objects, we provide photoresistors based onsemiconductor materials and specifically a mixture of cadmium sulfideand cadmium selenide crystals. The cadmium sulfide crystals initiallyhave a diameter of approximately 2 microns with the cadmium selenideparticles being -200 microns in diameter. We have found that thephotoresistor rise time is greatly influenced by film thicknesses aswell as film composition. We have found an optimum film thickness rangeof between 5 to 10 microns as being advantageous with 6 to 8 microns.The thin films may be deposited by employing silk-screening techniquesand by carefully controlling the vehicle viscosity and silk-screen meshsize. We have also produced acceptable thin films by air spraying thephotoresistor mix and the vehicle. Other printing techniques may also beemployed.

We have, furthermore, found that sintering temperatures and sinteringtimes also afiect rise times. Also important in the preparation of aphotoresistor having a short rise time is homogeneity of thephotoresistor mix. This also includes the content with respect toactivators. Inasmuch as the activator concentration and ultimatephotoconductive mix affects the photoresistivity including the darkresistivity, we have found that the composition of the mix prior to, aswell as after sintering, must be closely controlled to effect thedesired results. We have, furthermore, found that the spectral responseis directly related to the cadmium sulfide-cadmium selenide basicmixture, as well as the use of various activator materials. Furthermore,we have found that the film thickness also greatly affects spectralresponse. Especially significant insofar as the slope of the log ofphotoresistance versus the log of illumination, is the copper activatorand the cadmium chloride concentration.

The invention may be better understood from the following illustrativedescription and the accompanying drawings.

FIG. 1 of the drawings is a rise time plot between illumination andtime.

FIG. 2 is a spectral response plot of a typical photoresistormanufactured in accordance with the present invention.

FIG. 3 is a plot between log photoresistance and log illumination oftypical photoresistors in accordance with the present invention.

FIG. 4 illustrates the structure of a photoresistor of the presentinvention.

In FIG. 1 is illustrated a typical rise time curve for the devices ofthe invention where a rise time of about 5 milliseconds or less issecured in response to a change in illumination of from about 0.2 toabout 25 ft. candles. The time taken for the resistance to change 63% ofthe total resistance change is a measure of the response time of thephoto device and involves, as shown in the plot, the time for theresistance to reach 1/ e of its final value, where e is the well knownconstant associated with Naperian logarithms. Another plot isillustrated in FIG. 2 wherein the spectral response of the instantdevices is shown. Readily apparent is the significant improvement overprior art photoresistors and the nearness of the instant invention to anideal photoresistor. Shifting of the peak of the response to suit thedesired wavelength is realized by carefully controlling the compositionin accordance with the inventive concept herein, as well as its methodof preparation in films of specific thicknesses. Additions of CdSe shiftthe response peak toward longer wavelengths.

FIG. 3 describes devices of the invention by utilizing a plot betweenillumination and photoresistance wherein the slope of the curve is foundto represent a value between about 0.85 and 1.0 for low illuminationlevels. The observed slope for higher illumination levels is slightlyless with a slope of 0.82 having been realized. For Cu additionsreaching up to about 1,000 p.p.m. the log-photoresistance versuslog-illumination slope is directly influenced and increases to a valueof about 1.8 whereas lower Cu concentrations involving only a few p.p.m.have a smaller slope value. Critical control of the CdSe to CdS ratiohas been found necessary since high ratios effect a decreasing slopevalue. Similarly, CdCl additions additions adversely affect the slopevalues. We have found that the devices of the invention, as noted in thetwo curves having higher slopes as well as higher photoresistance, aresuperior to those of the prior art wherein slopes of 0.8 are typical. Wehave also found that even higher slope values can be obtained bycritical control of film thickness, with thin film thicknesses alsobeing less sensitive to temperature changes.

FIG. 4a is a top view of a photoresistor mounted within a hermeticallysealed metal can I positioned underneath a glass window 7. In FIG. 4b, across-sectional view, there is shown an active CdS-CdSe film 3 which isdeposited, preferably by silk-screening or spraying, onto an aluminadisc 4, or the substrate may be other suitable insulating material suchas polymeric, refractory organic, or various vitreous materials. Aftersintering of the composite, suitably conducting contacts 2 such as tinare evaporated through a mask onto the film 3, with external electricalconnections being suitably provided through wires 8 extending throughoptional insulating supporting means 5, disc 4, and film 3, to makeconnection with the finger shaped contacts 2. Vitreous materials havingsuitable transparencies to wavelengths of interest, as well as polymericmaterials, are especially desirable in lieu of the can typeencapsulation means for special applications and where low cost isimportant. Hermetic sealing of the device precludes the deteriorationnormally occurring during operation in even moderately humid ambients.Sealing of the external leads in the epoxy resin, the vitreous material,or other suitable encapsulation means, is also desirable to provide fora complete hermetic seal. The encapsulation means also enhances mountingease and prevents damaging the active film surface.

The photocells manufactured according to the invention have exceedinglyhigh photoconductivity and have likewise exceedingly fast rise times,broad spectral response, and are functionally stable over a period oftime. We have found that a specific narrow range of constituents willprovide the required combination of desirable device properties. Thiscomposition consists of 66.13 mol percent of CdS, 28.22 mol percent ofCdSe, 5.62 mol percent of CdCl -2 /2H O, and 0.03 mol percent CuCl -2HO. This mixture is provided in slurry form and is subsequently dried andground into fine particles. The mixture is then suspended in a 5% ethylcellulose toluene solu tion by ball milling for 30 minutes. The cadmiumsulfide-selenide mixture is approximately one-third the weight of thesuspending liquid. This suspension is then silkscreened onto aluminawafers and allowed to dry with subsequent baking at a temperature ofapproximately 550 C. to secure interdiffusion of consituents and filmhomogeneity.

We have found that one other narrow range of constituents will providefor the desirable combination of photoconductive properties with othercompositions being undesirable. A specific composition within thisextremely narrow critical range consisted of 87 mol percent CdS, 7.3 molpercent CdSe, 5.7 mol percent CdCl and 0.035 mol percent CuCl Anotherphotoresistor in this range consisted of a completely diffusedhomogeneous film of 84 mol percent CdS, 10.6 mol percent CdSe, 5.9 molpercent CdCl and 0.035 mol percent CuCI We have found that the use ofcadmium selenide in a narrow range as an addition element to cadmiumsulfide photoresistors will function to decrease rise times and also hasa pronounced effect upon dark to light adaptation differences. Ourinvestigations suggest that cadmium selenide introduces traps in cadmiumsulfide which normally are nonexistent. This is reasonable since, notonly does the cadmium selenide act as a dopant, but boundaries betweencadmium sulfide and cadmium selenide granules can also introducetrapping centers. Furthermore, we have found that the oxidation behavioris different for cadmium sulfide and cadmium selenide, and would securea material containing surface energy levels different than in theabsence of cadmium selenide and its normal oxidation products.

We have found that the cooling rate after sintering of thephotoconductive film is important. Long cooling periods give betterresults in that strain-free granular materials are produced having fewerdefects with photoresistors having smaller adaptation differences.Prolonged cooling cycles however adversely affect rise time andtherefore are undesirable. We have found particularly effective acool-down cycle having a 15 to 20 minutes duration.

Radioactive isotopes effects suggest that alpha and/or beta, and a lowenergy gamma radiation can act as a biasing source to maintained filledtraps, and thereby function to negate the effects of having to fill andempty these traps as a condition of light change. This approach has beenespecially useful with CdS photodetectors and has functioned to decreasethe light-to-dark adaptation differences as well as to obtain improvedrise times. T1 as well as Ni have been incorporated in the mix as wellas secured in the form of a thin film painting on the inner periphery ofthe encapsulation means. When it is incorporated in the mix, the isotopewas less effective due to vaporization, whereas when incorporated in theencapsulation means, its utility is greater. Good results were obtainedby use of these isotopes, even in exceedingly small concentrations. Itis simple to determine the amount of activity needed for a particularmix since this can be readily determined by increasing the radioactiveisotope concentration until the dark resistance begins to reflect achange.

In the manufacturing processes of the invention, after the sinteringoperation, the fused thin photoconductor film is provided with aninterleaved finger type metallic contact structure on its surface.Exceedingly good results have been obtained by using transparentexceedingly thin contacts of indium and indium-tin oxides. Other thintransparent metallic contacts may also be employed. The device is thenprovided with a container by mounting into a TO-5 header with anappropriate Window, as noted in FIG. 4, or by sealing into a vitreousceramic or glass or transparent polymeric material.

A particularly desirable method of providing transparent low resistanceindium contacts involves the vapor diffusion of indium into the surfaceof the film. The diffusion step is accomplished under a positivepressure of hydrogen and by heating the assembly to a temperaturesufficient to provide a high concentration of indium atoms in thephotoconductive surface. It is found highly desirable to maintain aslight temperature gradient of approximately 5 C. per inch whichfunctions to keep the indium vapors from leaving the evaporationchamber. Masking, of course, is employed to provide for the desiredelectrode configuration. Another method for providing good ohmictransparent nonreflecting contacts utilizes an 82% indium, 18% tinalloy, that is reactively sputtered in an oxidizing ambient onto thefilm surface. Good results have been obtained with tin concentrations aslow as and as high as 30% with the balance being indium. Thetransmittance characteristics of a sputtered In O -Sn film provides foran almost 100% transmittance for the wavelengths of interest in theinvention.

While superior results are obtained with the specific mixtures disclosedabove, we have found that films doped with 100 ppm. of copper showspectral response peaking out at approximately 5200 A. By increasing thecopper concentration to 400 ppm, the response line is broadened with thepeak moving to 5500 A., sliver being useful but less effective.Additional cadmium selenide also functions in the same manner byshifting the peak toward longer wavelengths.

Preliminary evaluations show that the composition employed in theinvention is extremely useful for large area photo devices employable inintegrated circuits as well as in hybrid microcircuitry involving bothactive and passive components. Ease of preparing such active and passivecomponents allows for the providing of exceedingly low cost electricaldevices per unit function. Thin cadmium selenide films have also beenemployed for card reader logic circuitry with extremely good results.The photoresistor is also employable into magnetic tape controls as Wellas in character recognition equipments. Because of its extremely rapidrise times as well as photosensitivity, it is extremely useful in cameraaperture control mechanisms as Well as in various other visible andultra-violet region control mechanisms.

While cadmium sulfide single-crystals give better response in theultra-violet region, economics suggest the use of the invention wherelow cost is a factor in such applications as check-out equipment as wellas in various types of communication techniques employing light of theherein disclosed wavelengths. Its usefulness in various types of logiccircuits as well as in automatic counters is only limited by itsresponse'time which is about several milliseconds.

It may thus be seen that the invention is broad in scope and includessuch modifications as will be apparent to those skilled in the art, andwill be particularly apparout after benefiting from the teachings andequivalents disclosed herein and especially in view of those teachingsspecifically embraced by the instant invention. It is to be understoodthat the invention is not limited to the specific embodiments hereofexcepting as defined in the appended claims. Having thus described ourinvention, we claim:

1. A photoresistor comprising:

(a) an insulating substrate,

(b) a thin film of homogeneous photosensitive material contiguous withsaid substrate,

(c) said homogeneous photoconductive material consisting essentially ofto mol percent CdS, 20 to 30 mol percent CdSe, 1 to 8 mol percent CdCIand from 0.01 to 0.05 mol percent CuCl (d) spaced metal electrodesconnected to said thin film of photosensitive material,

(e) encapsulation means completely surrounding said photoresistorexcepting for exposed electrical conducting means making connection tosaid thin film of photosensitive material.

2. A photoresistor according to claim 1 wherein said homogeneousphotosensitive material is 6 to 8 microns in thickness.

3. A photoresistor according to claim 1 wherein the insulating substrateis a thin A1 0 wafer.

4. A photoresistor according to claim 1 wherein the spaced metalelectrodes making electrical connection to said thin film ofphotoconductive material are a transparent indium compound.

5. A photoresistor according to claim 1 wherein the encapsulation meanscomprises a low temperature vitreous ceramic.

6. A photoresistor according to claim 1 wherein the encapsulation meanscomprises transparent polymeric material.

7. A photoresistor according to claim 1 wherein the encapsulation meanscomprises a metal container having a transparent window mounted therein.

8. A photosensitive device comprising:

(a) an insulating substrate,

(b) a thin film of homogeneous photoconductive material overlying saidsubstrate, consisting essentially of 84 to 87 mol percent CdS, 7 to 11mol percent CdSe, 5 to 7 mol percent CdCl and 0.02 to 0.4 mol percentCuCl (c) spaced electrodes overlying said photoconductive material,

(d) encapsulating means surrounding said substrate, photoconductivematerial, and spaced electrodes, and

(e) external means making electrical connection to said spacedelectrodes.

9. A device according to claim 8 wherein said thin film of homogeneousphotoconductive material is 6 to 8 microns in thickness.

10. A device according to claim 8 wherein said spaced electrodes consistof a transparent indium compound.

11. A photosensitive device comprising:

(a) an insulating substrate,

(b) a thin film of homogeneous photoconductive material composed of 60to 87 mol percent CdS, 7 to 30 mol percent CdSe, 1 to 8 mol percent CdCland 0.01 to 0.05 mol percent CuCl (c) spaced electrodes overlying saidphotoconductive material, and

(d) encapsulating means completely surrounding said device excepting forexposed electrical conducting means making electrical connection to saidphotoconductive material.

12. A photosensitive device according to claim 11 wherein said thin filmof homogeneous photoconductive material contains radioactive isotopes.

13. A photosensitive device according to claim 11 7 8 wherein saidencapsulation means contains a thin film of wherein said thin film ofhomogeneous photoconductive radioactive isotope material securedthereto. material is 6 to 8 microns thick.

14. A photosensitive device according to claim 11 wherein said spacedelectrodes are a transparent indium compound.

15. A photosensitive device according to claim 11 5 RICHARD WOOD PrimaryExaminer No references cited.

1. A PHOTORESISTOR COMPRISING: (A) AN INSULATING SUBSTRATE, (B) A THINFILM OF HOMOGENEOUS PHOTOSENSITIVE MATERIAL CONTIGUOUS WITH SAIDSUBSTRATE, (C) SAID HOMOGENEOUS PHOTOCONDUCTIVE MATERIAL CONSISTINGESSENTIALLY OF 60 TO 70 MOL PERCENT CDS, 20 TO 30 MOL PERCENT CDSE, 1 TO8 MOL PERCENT CDCL2, AND FROM 0.01 TO 0.05 MOL PERCENT CUCL2, (D) SPACEDMETAL ELECTRODES CONNECTED TO SAID THIN FILM OF PHOTOSENSITIVE MATERIAL,(E) ENCAPSULATION MEANS COMPLETELY SURROUNDING SAID PHOTORESISTOREXCEPTING FOR EXPOSED ELECTRICAL CONDUCTING MEANS MAKING CONNECTION TOSAID THIN FILM OF PHOTOSENSITIVE MATERIAL.